There is growing interest in Ocean Thermal Energy Conversion (“OTEC”) systems as a means of carbon-free electric power generation. OTEC systems exploit the temperature difference between the warm surface waters of tropical seas and the cold waters of the deep ocean to generate electricity.
An OTEC system, such as closed-loop OTEC system 100 depicted in FIG. 1, usually resides on floating platform, ship, or barge 102. Key components of system 100 include closed-loop conduit 104, evaporator 106, warm water pipe 108, turbo-generator(s) 110, condenser 112, and cold water pipe 116.
Closed-loop conduit 104 contains working fluid 103, such as ammonia, etc. In operation, the liquid working fluid is pumped to evaporator 106. Warm surface water is also conducted to the evaporator via warm water pipe 108. Heat transferred from the water vaporizes the low-boiling point working fluid.
The vaporized working fluid flows to turbo-generator 110, where it is used to rotate a turbine. The turbine, in turn, drives an electrical generator to produce electrical energy. After the vaporized working fluid transits turbo-generator 110, it is condensed in condenser 112. Condensation is effected using cold sea water piped up from the deep ocean via cold water pipe 116. The now-liquid working fluid is pumped to evaporator 106 via pump 114 and the cycle continues.
Although conceptually quite simple, an OTEC system presents certain manufacturing challenges. Consider, for example, cold water pipe 116. To retrieve cold water, the cold water pipe extends vertically downward into the ocean about 1000 meters or more. As a consequence of the small temperature differential between the cold and warm waters that drives the OTEC process, this pipe must convey an exceedingly large quantity of water to the condenser to meet its duty requirements. Consequently, in addition to its extraordinary length, the cold water pipe must have a very large diameter. In fact, for a commercial-scale plant, the cold water pipe is likely to have a diameter of about 30 feet. To fabricate, transport, and install such pipe is a substantial undertaking.
Due to certain advantageous properties compared to metals (e.g., lighter weight, greater strain tolerance, better corrosion resistance, etc.) a polymer-matrix continuous-fiber composite material is potentially a good material from which to fabricate the cold water pipe. A variety of processes are available for producing suitable composite materials.
At 1000 meters or more in length, an OTEC cold water pipe is far too long to be molded all at once (i.e., “in one shot”) in a single production operation. Rather, if made by ordinary methods, the cold water pipe would be formed as discrete lengths of pipe by molding a plurality of discrete, shorter pipe sections that are then connected by mechanical joints or adhesive bonding. Examples of other pipe-like items that are typically fabricated as a plurality of discrete lengths joined together mechanically include smokestacks, tunnel liners, and previous attempts to create OTEC cold water pipes in smaller-scale work.
Although discrete lengths can be used to form the cold water pipe, the resulting joints are not as strong as if the laminate were continuous across the joints. In other words, the pipe would be stronger if there were no joints. And these joints exhibit other disadvantages as well, including increased weight, complexity, and lower reliability than the basic composite material. Furthermore, there is considerable difficulty and expense to transporting 1000 meters worth of 30-foot diameter pipe over land and water to its destination (i.e., the floating platform). And that is the case regardless of whether the pipe is transported as a plurality of discrete segments that are to be assembled at the floating platform or whether they are preassembled and transported as a single (exceedingly long) unit that must then be “upended” into vertical position and attached to the platform.
The fabrication of an OTEC cold water pipe is therefore a challenging task for which no satisfactory approach currently exists.